Dual or multi-energy spectral computed tomography (CT) systems can reveal the densities of different materials in an object and generate images acquired at multiple monochromatic x-ray energy levels. In the absence of object scatter, a system derives the behavior at a different energy based on a signal from two regions of photon energy in the spectrum: the low-energy and the high-energy portions of the incident x-ray spectrum. In a given energy region of medical CT, two physical processes dominate the x-ray attenuation: Compton scattering and the photoelectric effect. The detected signals from two energy regions provide sufficient information to resolve the energy dependence of the material being imaged. Detected signals from the two energy regions provide sufficient information to determine the relative composition of an object composed of two hypothetical materials.
However, in some cases the detected signals may not provide sufficient information to resolve the energy dependence of the material being imaged due to low photon flux, or photon starvation, when the attenuated x-ray beam at the detectors is weak. For example, photon starvation may occur for low energy x-ray beams due to a reduced number of photons generated compared to high energy x-ray beams. As a result, low energy data may be noisier and less reliable than high energy data, which may in turn cause substantial artifacts in an image reconstructed from the data. This problem with low energy data may be further exacerbated by sparse view data acquisition, where data is acquired at fewer views and therefore there is less low energy data overall.